PR22763: if a defaulted (non-user-provided) special member function is

explicitly instantiated, still emit it with each use.

We don't emit a definition of the member with an explicit instantiation
definition (and indeed it appears that we're not allowed to, since an explicit
instantiation definition does not constitute an odr-use and only odr-use
permits definition for defaulted special members). So we still need to emit a
weak definition with each use.

This also makes defaulted-in-class declarations behave more like
implicitly-declared special members, which matches their design intent.
And it matches the way this problem was solved in GCC.

llvm-svn: 318474

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 8247d1a23ef..d3a5f075dc2 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -8963,6 +8963,12 @@ static GVALinkage basicGVALinkageForFunction(const ASTContext &Context,
   if (!FD->isExternallyVisible())
     return GVA_Internal;
 
+  // Non-user-provided functions get emitted as weak definitions with every
+  // use, no matter whether they've been explicitly instantiated etc.
+  if (auto *MD = dyn_cast<CXXMethodDecl>(FD))
+    if (!MD->isUserProvided())
+      return GVA_DiscardableODR;
+
   GVALinkage External;
   switch (FD->getTemplateSpecializationKind()) {
   case TSK_Undeclared: